1. Field of the Invention
This invention relates to data processing systems and more particularly to the use of commands on a system bus which are sent by one subsystem to tell another subsystem to send a response command to a third subsystem.
2. Description of the Prior Art
U.S. Pat. No. 4,030,075 entitled "Data Processing System Having Distributed Priority Network", invented by George J. Barlow, describes a data processing system in which a number of subsystems are coupled in common by a system bus. The subsystems communicate with each other by sending commands out on the system bus. A command may include a channel number identifying a receiving unit, a channel number identifying a sending subsystem and a function code specifying an operation to be performed by the receiving subsystem. The function code may require that the receiving subsystem generate a response command. This response or second half bus cycle command includes the channel number identifying the sending subsystem. The sending subsystem responds to its channel number and acknowledges the receiving of the second half bus cycle command.